1. Field of the Invention
The present invention relates to a gas sensor installed in a flow path through which flows a gas to be measured, such as an exhaust gas, the gas sensor functioning while introducing reference air into the gas sensor from its exterior.
2. Description of the Related Art
Conventionally known gas sensors for determining the concentration of a specific gas component in a mixed gas include HC sensors and NOx sensors. An example of such a gas sensor is an oxygen sensor as shown in FIG. 3 of the accompanying drawings. The oxygen sensor includes a detection element S, which, in turn, includes an oxygen-ion-conductive solid electrolyte body.
The detection element S includes an element body 503, which is a closed-bottomed cylindrical solid electrolyte body which is closed at one end and opened at the other end; an internal electrode 505 formed on the inner surface of the element body 503; and an external electrode 507 formed on the outer surface of the element body 503. An electromotive force is generated between the internal and external electrodes 505 and 507 according to the difference in oxygen concentration between the internal space of the detection element S and the exterior of the detection element S. For example, when the closed end of the detection element S is projected via a metallic shell 509 into the interior of the exhaust pipe of an internal combustion engine to thereby expose the external electrode 507 to an exhaust gas, the oxygen concentration in the exhaust gas can be determined from an electromotive force generated between the internal and external electrodes 505 and 507.
In order to accurately measure the oxygen concentration in a gas being measured, a reference oxygen concentration on the side of the internal electrode 505 must be maintained constant. Thus, for example, air is introduced into the interior of the oxygen sensor through gaps among strands of a lead wire. However, when foreign matter, such as water, enters the internal space of the detection element S, the oxygen concentration cannot be measured properly.
To cope with this problem, conventionally, as shown in FIG. 3, a cylindrical cover (an internal cylindrical member 511 and an external cylindrical member 513) is attached to the upper opening (i.e., the opening located outside the flow path) of the metallic shell 509 so as to cover an upper portion of the detection element S. Also, a rubber member 515 is disposed at an exit portion of the cylindrical cover for lead wires connected to the internal and external electrodes 505 and 507 in order to prevent foreign matter, such as water, from entering the internal space of the detection element S.
In spite of employing of the above-mentioned waterproofing measures, entry of the gas being measured from the flow path or capillarity-effected entry of liquid (for example, water or oil) through gaps among strands of a lead wire cannot be prevented completely. A gas generated through vaporization of liquid which has entered, or a gas being measured which has entered may cause a change in the oxygen concentration within the oxygen sensor, thus affecting the results of the measurement. To cope with this problem, for example, a gas-permeable water-repellent filter 517 of a fluorine-containing resin is disposed to cover intercommunicating holes 519 and 521 formed in the wall of the cylindrical cover, whereby a gas which may cause measurement error is ejected therethrough, and air is introduced therethrough into the oxygen sensor, to thereby measure the oxygen concentration accurately.
When the above-mentioned oxygen sensor is used for determining the oxygen concentration in a high-temperature exhaust gas, the oxygen sensor is placed in a high-temperature environment. Thus, the metallic shell 509, which is attached to an exhaust pipe while holding the detection element S, and the internal and external cylindrical members 511 and 513, which are adapted to supply air to the internal electrode 505, are formed from a metal (for example, stainless steel).
However, the rubber member 515 and the water-repellent filter 517 are low in critical heat-resistant temperature as compared with a refractory metal. Thus, long-term use at high temperature has occasionally involved the following phenomena: the elasticity of the rubber member 515 deteriorates, thereby causing an impairment in waterproofing; and the water-repellent filter 517 is thermally deformed, thereby causing blockage of holes with a resultant impairment in gas permeability.
When the oxygen sensor is attached to the exhaust pipe of a car, a certain position of attachment may involve splashing the oxygen sensor with engine oil. In this case, oil may enter the interior of the oxygen sensor through the water-repellent filter 517, thereby impairing the function of the oxygen sensor or impairing the gas permeability of the water-repellent filter 517 with a resultant difficulty in introducing air in a favorable manner.
The above-mentioned problems arise with not only an oxygen sensor but also other gas sensors which require waterproofing and introduction of air, such as NOx sensors and HC sensors.
The present invention has been accomplished in view of the above-mentioned problems, and an object of the present invention is to improve the heat resistance and oil resistance of a gas sensor.
Accordingly, a gas sensor of the present invention is characterized as comprising: a detection element comprising an oxygen-ion-conductive solid electrolyte body and electrodes formed on the solid electrolyte body; a metallic shell formed so as to be attachable in a mounting hole formed in a wall of a flow path through which flows a gas to be measured, and adapted to hold the detection element; a cylindrical cover disposed opposite the flow path with respect to the metallic shell and adapted to introduce air to one of the electrodes formed on the solid electrolyte body, within which lead wires electrically connected to the electrodes of the detection element are disposed and at least a portion of which is bendable; an elastic member having an intercommunicating hole formed therein through which air is introduced into the interior of the cylindrical cover, and having through-holes formed therein through which the lead wires extend; and a water-repellent filter having gas permeability and positioned such that air to be introduced into the interior of the cylindrical cover through the intercommunicating hole formed in the elastic member passes therethrough.
In the gas sensor of the present invention having the above-mentioned structure, the cylindrical cover is disposed opposite the flow path with respect to the metallic shell which holds the detection element. The elastic member for waterproofing is disposed at an opening portion of the cylindrical cover located opposite the metallic shell. The elastic member has through-holes formed therein. The lead wires extend to the exterior of the gas sensor through the through-holes. Also, the elastic member has formed therein the intercommunicating hole through which air is introduced into the interior of the cylindrical cover. The water-repellent filter is disposed such that air to be introduced into the cylindrical cover through the intercommunicating hole passes therethrough. These structural features enable introduction of air into the interior of the cylindrical cover and prevent entry of water into the same.
Particularly, in the present invention, at least a portion of the cylindrical cover is bendable. By bending the bendable portion, the position of the elastic member can be changed.
Thus, when the gas sensor of the present invention is installed where the gas sensor is potentially susceptible to oil splashes, the elastic member having the intercommunicating hole formed therein can be moved to such a position at which the elastic member is less susceptible to oil splashes, through bending and straightening the cylindrical tube.
Accordingly, the possibility of entry of oil into the gas sensor, which causes a detection error, or the possibility of adhesion of oil to the water-repellent filter, which hinders gas flow through the intercommunicating hole (introduction of air from outside the gas sensor or ejection of the exhaust gas which has entered the interior of the gas sensor), can be reduced.
According to the gas sensor of the present invention, the cylindrical cover is bendable, so that the cylindrical cover can be disposed along the route of the lead wires. Specifically, the length of the cylindrical cover can be increased so long as an ambient space of the installed gas sensor permits. Thus, in the case where the gas sensor is attached to an exhaust pipe, heat transmitted from the metallic shell to the elastic member through the cylindrical cover can be decreased. Also, the elastic member can be moved to a position less exposed to radiant heat from the exhaust pipe. As a result, a temperature rise of the elastic member and the water-repellent filter can be suppressed, thereby extending their service lives and thus extending the service life of the gas sensor.
When the gas sensor is used to determine the concentration of a target component in a high-temperature exhaust gas, the temperature of the metallic shell used to attach the gas sensor to an exhaust pipe, through which the exhaust gas flows, becomes high. Thus, the temperature of the cylindrical cover attached to the metallic shell also becomes high in the vicinity of the metallic shell.
To cope with the above problem, advantageously, the cylindrical cover may comprise a first cover formed from a metal and disposed opposite the flow path with respect to the metallic shell; and a second cover being bendable and disposed opposite the metallic shell with respect to the first cover.
According to the gas sensor having this structure, a portion of the cylindrical cover located in the vicinity of the metallic shell is composed of the first cover of a metal, thereby improving the heat resistance of the cylindrical cover.
In the above preferred gas sensor, the bendable second cover disposed opposite the metallic shell with respect to the first cover may assume the form of cylindrical bellows made of a metal, thereby reliably protecting the lead wires accommodated therein. However, when the first and second covers are of a metal, the gas sensor may become too heavy, potentially causing inconvenience in handling. Also, when the first cover and the second cover differ in coefficient of thermal expansion, the connection of the first and second covers may involve difficulty in maintaining good sealing performance.
To cope with the above problem, preferably, the second cover may be formed from an elastic, heat-resistant resin or rubber.
According to the gas sensor having this structure, the second cover is formed from a resin or rubber, so that the total weight of the oxygen sensor can be decreased as compared to the case where the second cover is formed from a metal. Also, the elasticity of the second cover facilitates maintenance of a seal at a connection with the first cover, thereby yielding a waterproofing effect. Since a resin or rubber is poorer in thermal conductivity than a metal, a temperature rise of the elastic member and the water-repellent filter can be further suppressed. Examples of an elastic, heat-resistant resin include PTFE resins, such as Teflon (trade name), and PFA resins. The second cover of a fluorine-containing resin exhibits excellent heat resistance and oil resistance and is thus preferred. Specific examples of rubber include silicone rubber, EPDM rubber, and composite rubber of silicone rubber and other rubber material.
The water-repellent filter may assume the form of a hard, porous foamed body of a PTFE resin inserted into the intercommunicating hole formed in the elastic member. However, this form is disadvantageous in terms of intercommunicating performance.
To cope with this problem, preferably, the water-repellent filter may assume the form of a sheet and may be disposed so as to cover an opening of the intercommunicating hole facing the exterior of the gas sensor.
According to the gas sensor having this structure, the water-repellent filter assumes the form of a sheet, so that good intercommunication is established; specifically, air can be smoothly introduced into the interior of the gas sensor, and an exhaust gas which has entered the interior of the gas sensor can be smoothly ejected to the exterior of the gas sensor.